Search results for "bond"
showing 10 items of 3527 documents
Do the Intramolecular π Interactions Improve the Stability of Ionic, Pyridine-Carbene-Based Iridium(III) Complexes?
2013
Throughout the last years one of the most intensive research topics in light-emitting electrochemical cells (LECs) focused on the design of blue-emitting, ionic iridium(III) complexes. To this end, the most recent strategy is the use of carbene-based ancillary ligands. Although blue LECs have been successfully fabricated, the stability has been noted as the main drawback. To overcome this problem, Zhang et al. have recently explored the use of π interactions to enhance the strength of pyridine-carbene-based complexes. The authors suggested that the use of intramolecular π–π stacking interactions by means of pendant phenyl rings to improve the stability of LECs is not as effective as in devi…
Magnetostructural correlations in parallel square-planar halo-bridged copper(II) dimers. Part II: Structure and magnetic properties of [Cu2(terpy)2Br…
1987
Abstract The crystal and molecular structure of bis-μ- bromo-bis [(2,2′:6′,2″ - terpyridyl) copper (II) ] bis (hexafluorophosphate) has been determined from three dimensional single-crystal X-ray diffraction data, collected by counter techniques. The blue crystals belong to the monoclinic system, space group P21/a, with four formula units in a unit cell of dimensions a=20.40(2), b=13.35(2), c=6.453(8) A and β=97.74(9)°. The structure was solved using a refinement procedure and starting with the atomic positions of the isostructural complex [Cu(terpy)- Cl]2(PF6)2, to final agreement factors R=0.071 (Rw=0.077). The structure consists of dimeric molecules bridged by two bromine atoms. The coor…
Localized molecular orbitals for excited states of polyenals, polyendials, and polyenones
2003
The work is focused on the generation of localized molecular orbitals for excited states. A recently developed a priori method based in a CAS-SCF–type algorithm is applied. The method generates directly localized orbitals and can be applied to multireference wavefunctions. A detailed description of the performance of the method as well as the locality of the MOs for the example of the singlet nπ* (CO) excited state is given. It is in general possible to obtain local orbitals for the doubly occupied and virtual valence orbitals. The partial delocalization of the π* (CO) orbital is discussed, as is the effect of the use of different CAS spaces. The systems under study are polyenals, polyendia…
Anion ordering and defect structure in Ruddlesden-Popper strontium niobium oxynitrides
2004
The crystal structure of the n = 1 member of the Ruddlesden-Popper family (SrO)(SrNbO(2)N)(n) was refined by the Rietveld method using neutron powder diffraction data. This complex crystallizes in the I4/mmm space group with cell parameters a = 4.0506(2) and c = 12.5936(9) angstroms. The refined composition was Sr(2)NbO(3.28)N(0.72), which corresponds to a formal oxidation state for Nb of +4.72, meaning 72% Nb(V) and 28% Nb(IV). The nitrogen atoms order in the equatorial sites of the niobium octahedra according to Pauling's second crystal rule as the more charged anion occupies the site showing the larger bond strength sums. Pauling's second crystal rule is shown to be able to predict the d…
DFT Calculations of the Electric Field Gradient at the Tin Nucleus as a Support of Structural Interpretation by119Sn Mössbauer Spectroscopy
2005
DFT calculations, using an all-electron basis set and with full geometry optimization, were performed on 34 Sn(II) and Sn(IV) compounds of known structure and (119)Sn Mössbauer parameters, to obtain the theoretical values of the electric field gradient components, V(xx), V(yy), and V(zz), at the tin nucleus. These were used to determine the quantity V = V(zz)[1+ 1/3((V(xx) - V(yy))/((V(zz))(2)](1/2), for each investigated compound, which is related to the quadrupole splitting (DeltaE) parameter according to DeltaE = 1/2eQV, where e is the electronic charge and Q is the quadrupole moment of the tin nucleus. The linear fitting of the correlation plot of the experimental DeltaE, versus the cor…
Dynamic Molecular Graphs: “Hopping” Structures
2013
This work aims to contribute to the discussion about the suitability of bond paths and bond-critical points as indicators of chemical bonding defined within the theoretical framework of the quantum theory of atoms in molecules. For this purpose, we consider the temporal evolution of the molecular structure of [Fe{C(CH2 )3 }(CO)3 ] throughout Born-Oppenheimer molecular dynamics (BOMD), which illustrates the changing behaviour of the molecular graph (MG) of an electronic system. Several MGs with significant lifespans are observed across the BOMD simulations. The bond paths between the trimethylenemethane and the metallic core are uninterruptedly formed and broken. This situation is reminiscen…
Large Quadratic Hyperpolarizabilities with Donor–Acceptor Polyenes Exhibiting Optimum Bond Length Alternation: Correlation Between Structure and Hype…
1997
Donor-acceptor polyenes of various lengths, and that combine aro- matic electron-donating moieties with powerful heterocyclic electron-withdraw- ing terminal groups, have been synthe- sized and characterized as efficient non- linear optical (NLO) chromophores. Their linear and nonlinear optical proper- ties have been investigated, and variations in these properties have been related to ground-state polarization (dipole p) and structure. In particular, unprecedented quadratic hyperpolarizabilities (p) have been achieved (up to p(0) =I500 x 10-30esu) by reduction of the bond- length alternation (BLA) in the polyenic chain. In each series of homologous com-
Synthesis and Characterisation of a New Series of Bistable Iron(II) Spin-Crossover 2D Metal-Organic Frameworks
2009
Twelve coordination polymers with formula {Fe(3-Xpy)(2)[M(II)(CN)(4)]} (M(II): Ni, Pd, Pt; X: F, Cl, Br, I; py: pyridine) have been synthesised, and their crystal structures have been determined by single-crystal or powder X-ray analysis. All of the fluoro and iodo compounds, as well as the chloro derivative in which M(II) is Pt, crystallise in the monoclinic C2/m space group, whereas the rest of the chloro and all of the bromo derivatives crystallise in the orthorhombic Pnc2 space group. In all cases, the iron(II) atom resides in a pseudo-octahedral [FeN(6)] coordination core, with similar bond lengths and angles in the various derivatives. The major difference between the two kinds of str…